As a method for manufacturing an SOI substrate, there are known the SmartCut method, the SiGen method, and the like based on conventional bonding.
The SmartCut method is a method in which a silicon substrate, on the bonding side of which hydrogen ions have been implanted, and a substrate made also of silicon or of another material are bonded together and subjected to a heat treatment at a temperature of 400° C. or higher (for example, 500° C.). Then, a silicon thin film is thermally peeled off from a region where the concentration of the implanted hydrogen ions is highest, thus obtaining an SOI substrate (see, for example, Japanese Patent No. 3048201 (patent document 1) and A. J. Auberton-Herve et al., “SMART CUT TECHNOLOGY: INDUSTRIAL STATUS of SOI WAFER PRODUCTION and NEW MATERIAL DEVELOPMENTS” (Electrochemical Society Proceedings Volume 99-3 (1999) pp. 93-106) (non-patent document 1)).
The SiGen method is a method in which before a silicon substrate, on the bonding side of which hydrogen ions have been implanted, and a substrate made also of silicon or of another material are bonded together, both or either one of the bonding surfaces of these substrates is plasma-treated. The two substrates are then bonded together with the surfaces thereof activated. After heat-treating the bonded substrates at a low temperature (for example, 100 to 300° C.) and thereby increasing the bonding strength thereof, a silicon thin film is mechanically peeled off at room temperature, thus obtaining an SOI substrate (see, for example, the specification of U.S. Pat. No. 6,263,941 (patent document 2), the specification of U.S. Pat. No. 6,513,564 (patent document 3), and the specification of U.S. Pat. No. 6,582,999 (patent document 4)).
The difference between these two methods mainly lies in the process of silicon thin film separation. The SmartCut method requires processing at a high temperature for the purpose of silicon thin film separation, whereas the SiGen method makes this separation feasible at room temperature.
As a rule, a silicon substrate and a substrate made of another material are bonded together in the manufacture of a bonded SOI substrate. These materials of different types generally differ from each other in the rate of thermal expansion, intrinsic allowable temperature limits, and the like. Consequently, if the temperature of heat treatment applied to the substrates being bonded in a manufacturing process becomes higher, the substrates are more likely to cause breakage or local cracks due to a difference in thermal properties between the both substrates. From this point of view, the SmartCut method which requires high temperatures for silicon thin film separation can hardly be said preferable as a method for manufacturing an SOI substrate based on the bonding of substrates made of different materials.
On the other hand, the SiGen method capable of low-temperature separation is less likely to cause breakage or local cracks due to the aforementioned difference in thermal properties. However, this method in which a silicon thin film is peeled off mechanically has the problem that it is more likely that the bonded (jointed) surfaces of the substrates separate from each other, traces of separation arise, or mechanical damage is introduced into the silicon thin film during a separation process.
The present invention has been accomplished in view of the above-described problems. It is therefore an object of the present invention to avoid breakage, local cracks and the like due to a difference in thermal properties between the substrates and the introduction of mechanical damage into an SOI layer in a step of manufacturing an SOI substrate by bonding together a single-crystal silicon substrate and a transparent insulating substrate, thereby providing an SOI substrate having an SOI layer superior in film uniformity, crystal quality, and electrical characteristics (carrier mobility and the like).